Ionizing radiation is a known environmental mutagen and carcinogen. These experiments use shuttle vectors to obtain precise information about the genetic effects of ionizing radiation on human cells. We focus on vectors based on Epstein-Barr virus (EBV), with the lacI gene of E. coli as the mutational target. These vectors resemble chromosomes and have an extremely low spontaneous mutation frequency. Yet, they can be easily retrieved to bacteria for rapid detection and analysis of mutations. Three central questions will be addressed. First, we will determine the mutagenic specificity of ionizing radiation in human cells at the DNA sequence level. A range of dose conditions will be used. Our second objective is to use the shuttle vector system to identify treatments that can counteract some of the mutagenic effects of ionizing radiation. We will focus on chemicals that have antimutagenic effects against the reactive oxygen species generated by ionizing radiation. Finally, we will examine with shuttle vectors two radiation-sensitive human genetic diseases, xeroderma pigmentosum (XP) and ataxia telangiectasia (AT). XP cells are sensitive to ultraviolet (UV) radiation. The vectors will be used to characterize the effects of UV radiation on cells from various XP complementation groups. AT cells are sensitive to ionizing radiation. The response of these cells to ionizing radiation will be followed using the vectors. With this program of experiments we hope to generate data not readily obtainable by previously existing techniques and to create a clear picture of the effects of ionizing radiation on human cells.